Abstract out curve25519 tests

Also rename the internal variable in crypto.js to be a little more
explicit about which curve we're dealing with.
Abstract nativeclient callback logic

The nativeclient.js module overrides
window.textsecure.registerOnLoadFunction with its own version. Otherwise
helpers will define a trivial placeholder for same.

The flag textsecure.NATIVE_CLIENT can be set anywhere ahead of
nativeclient.js, but is only acted on in nativeclient.js,
and crypto.js.
Wrap some long lines